In applications of this kind, one is generally confronted with noise-affected input voltages. For example, line voltages of the above mentioned AC grids are subject to frequency, amplitude and/or phase noise, as well as harmonics. If the zero crossings of the input voltage, i.e. in this case the grid voltage, are now used e.g. for operating a circuit arrangement in synchronism with the grid voltage, due to superimposed harmonics, for example, zero crossings which do not correspond to those of the fundamental component may be detected, possibly resulting in defective operation of the grid-synchronous circuit arrangement which may eventually cause spurious shutdowns of the grid-synchronous circuit.
Conventionally, high-frequency input voltage noise such as harmonics, for example, is eliminated by low pass filters. However, low pass filters produce a phase shift in the low pass filter's output voltage compared to the input voltage, which in turn cannot be simply corrected because of unknown input voltage frequency and phase disturbances. In addition, it must be taken into account that a reference signal of an external signal generator cannot be used to match the input voltage to the reference signal, for example. Instead, the reconstructed output voltage must correspond in frequency, amplitude and phase to the fundamental component of the input voltage, and not to a generated reference signal of an external signal generator.